Conventionally, as discharge lamps, there have been known discharge lamps that include a discharge container enclosing a halogen gas and a pair of electrodes placed to oppose each other within the discharge container. Further, as a discharge lamp lighting device for lighting such discharge lamps, there have been known a discharge lamp lighting device for supplying an AC electric current to discharge lamps (refer to Patent Document 1, for example).
When such a discharge lamp is lighted, the electrodes reach a higher temperature and are evaporated. Then, the evaporated metal bonds to the halogen gas enclosed within the discharge container and, thus, returns to the electrodes again without being adhered to the inner surface of the discharge container. This effect is referred to as a halogen cycle.
On the other hand, the discharge lamp lighting device according to Patent Document 1 is adapted to supply, to a discharge lamp, an AC electric current having two different frequencies independent of data about operations of the discharge lamp (voltage value, electric current value, luminance, distance between the electrodes, temperature and the like). By doing this, a shape of the electrodes can be preferably maintained at the beginning of lighting, but it has been difficult to maintain the shape of the electrodes (particularly, their protruding portions) as the lighting time period is increased. This has made the life of the discharge lamp shorter.